In wireless communication systems, the use of antenna arrays at the base station has been shown to increase both range, through increased gain, and capacity, through interference suppression. With adaptive antenna arrays, the signals received by multiple antenna elements are weighted and combined to improve system performance, e.g., by maximizing the desired receive signal power and/or suppressing interference. The performance of an adaptive antenna array increases dramatically with the number of antennas. Referring to an article entitled, "The Impact of Antenna Diversity on the Capacity of Wireless Communication Systems," by J. H. Winters, R. D. Gitlin and J. Salz, in IEEE Trans. on Communications, April 1994, it is shown that using an M element antenna array with optimum combining of the received signals can eliminate N.ltoreq.M-1 interferers and achieve an M-N fold diversity gain against multipath fading, resulting in increased range.
Most base stations today, however, utilize only two receive antennas with suboptimum processing, e.g., selection diversity where the antenna having the larger signal power is selected for reception and processing. It is desirable to be able to modify existing base stations to accommodate larger arrays of antennas and/or improved received signal combining techniques. However, modifying existing equipment is difficult, time consuming, and costly, in particular since equipment currently in the field is from a variety of vendors.
One alternative is to use a so called applique, which is an outboard signal processing box, interposed between the current base antennas and the input to the base station, and which adaptively weights and combines the received signals fed to the base station, optionally uses additional antennas. A key to the viability of using the applique approach is that it should require little, if any, modification of the base station equipment. This implies that the processing performed by the applique must be transparent to the existing equipment. Ideally, the signal emerging from the applique should appear to the existing base station as a high-quality received signal from a single antenna.
The signal processing functions performed by an adaptive array are typically designed to maximize the signal to interference-plus-noise ratio. One well known method for accomplishing this is by adjusting the adaptive array weights so as to minimize the mean squared error of the output signal with respect to a reference signal. Two common techniques for reference signal generation that are well known in the art are coherent reference and constant-modulus reference, the latter also being known in the art as the constant-module algorithm (CMA).
In light of the above considerations there is therefore a need for adaptive array weight generation that increases gain and improves interference suppression.